1. Field of the Invention
This invention relates to a process for producing a fuel oil base material and, more specifically, to a process for producing a fuel oil base material by hydrotreating a petroleum distillation residue having a relatively high sulfur content which is obtained from an atmospheric distillation unit or a vacuum distillation unit under specified conditions.
2. Background Art
Heretofore, in Japan, fuel oil has been produced by mixing as a main base material an atmospheric residue having a low sulfur content which is obtained by distilling crude oil having a low sulfur content in an atmospheric distillation unit to remove light hydrocarbons such as naphtha, kerosine or gas oil, or a vacuum residue having a low sulfur content which is obtained by further distilling this low-sulfur atmospheric residue in a vacuum distillation unit to remove vacuum gas oil with kerosine, gas oil or the like to control the viscosity thereof.
Meanwhile, from viewpoints of a short supply of low-sulfur crude oil, effective use of atmospheric or vacuum residues obtained from crude oil having a high sulfur content, and an increase in the production of a heart cut such as kerosine, gas oil or the like for the control of viscosity, hydrotreating processes for producing a fuel oil base material having a low sulfur content and low viscosity by contacting an atmospheric or vacuum residue obtained from crude oil having a high sulfur content with a hydrotreating catalyst at a high temperature and at a high hydrogen partial pressure to promote desulfurization, denitrification and cracking reactions have been developed and commercially operated.
Typical operation conditions for the hydrotreating processes include a reaction temperature of 350.degree. to 450.degree. C., a hydrogen partial pressure at an inlet of a reactor of 9.8 to 19.6 MPa, a liquid hourly space velocity of 0.1 to 5.0 h.sup.-1 and a hydrogen/oil ratio at an inlet of a reactor of 250 to 1,700 Nm.sup.3 /m.sup.3.
Although these hydrotreating processes, as described above, are extremely useful from viewpoints of a short supply of low-sulfur crude oil, effective use of atmospheric or vacuum residues obtained from crude oil having a high sulfur content, and an increase in the production of a heart cut such as kerosine, gas oil or the like for the control of viscosity, when a distillation residue is hydrotreated under high-severity operation conditions such as higher reaction temperature, dry sludge deposits in the product. The term "dry sludge" generally denotes particles essentially composed of asphaltene molecules having a diameter of 1.0 .mu.m or more.
When a base material having a high content of dry sludge is used as the base material of fuel oil, there is the possibility that it will grow into huge sludge when it is mixed with another base material or during the period when it is stored, causing such troubles as a blocked fuel oil filter or centrifugal oil purifier, fouling of a fuel oil heater, a choked fuel oil jet nozzle of a combustion engine, and the like.
Therefore, a hydrotreating process has so far been inevitably restricted by operation conditions such as a reaction temperature whose upper limit is that dry sludge does not deposit.
The reaction temperature at the start of operation is determined, taking into consideration an increase in the reaction temperature for compensating for a reduction in the activity of a hydrotreating catalyst used in the hydrotreatment of a distillation residue during operation because the activities of desulfurization, denitrification and cracking reactions generally deteriorate with the passage of operation time. The activity of the catalyst sometimes deteriorates faster than expected due to a change in type of stock oil typified by crude oil during operation, a change in the target value of the sulfur content of hydrotreated oil, or the like, and accordingly, a reaction temperature designed for the end of operation may be reached in the middle of operation.
Therefore, even if the reaction temperature at the start of operation is set to a temperature lower than a temperature at which dry sludge does not deposit, when a reaction temperature designated for the end of operation is reached in the middle of operation, dry sludge is produced. Therefore, the process has been restricted thereafter such that the conversions of desulfurization, denitrification and cracking reactions or the hydrotreatment rate of a vacuum residue that requires severe reaction conditions are reduced, only an atmospheric residue whose reaction conditions are relatively mild is hydrotreated, or the amount of the residue hydrotreated is reduced.